Method of testing the pressure integrity of a tank

ABSTRACT

A method of testing the pressure integrity of a tank containing hydrocarbon in liquid and gaseous phase and its associated pipework and valves, the tank having an inlet for the supply of liquid hydrocarbon to the tank, a first outlet for the removal of gaseous hydrocarbon from the tank, and a second outlet for the removal of liquid hydrocarbon from the tank; the method comprising the steps of closing in the inlet and first outlet and monitoring the pressure within the tank during normal operation of the tank as liquid hydrocarbon is removed from the second outlet.

[0001] The present invention relates to a method of testing the pressureintegrity of a tank and its associated pipework and valves containing ahydrocarbon in liquid and gaseous phase.

[0002] Such tanks are provided, usually underground, at filling stationsand are supplied with a liquid hydrocarbon fuel from a tanker. The tanksare linked to a number of dispensing pumps from which the fuel is drawnby motorists.

[0003] The tanks contain vapour which is vented from the tank via avapour return line to the delivery tanker which is simultaneouslysupplying liquid fuel to the tank. To prevent the vapour escaping toatmosphere, the vapour return line is fitted with a pressure/vacuum (P/Vvalve) which maintains a small backed pressure in the system. This,together with the small negative pressure in the tanker encouragesvapour to flow back to the tanker. The recovered vapour is taken away bythe tanker for recovery and re-use.

[0004] The tank and the associated system requires regular testing inorder to detect any leaks from the tank, pipework or valves which may becausing an environmental hazzard. Conventionally, this is done bysealing off the system, including an outlet from the tank to thedispensing pumps and artificially raising or lowering the containedpressure and monitoring how this varies over time. In this case, salesof the product from the site need to be halted and the cost of testingbecomes prohibitive discouraging environmental legislative compliance.

[0005] According to the present invention, there is provided a method oftesting the pressure integrity of a tank containing hydrocarbon inliquid and gaseous phase and its associated pipework and valves, thetank having an inlet for the supply of liquid hydrocarbon to the tank, afirst outlet for the removal of gaseous hydrocarbon from the tank, and asecond outlet for the removal of liquid hydrocarbon from the tank; themethod comprising the steps of closing the inlet and first outlet andmonitoring the pressure within the tank during normal operation of thetank as liquid hydrocarbon is removed from the second outlet.

[0006] The present invention works on the principle that by observing aknown amount of liquid fuel being withdrawn from the tank, it ispossible to obtain certain diagnostic information about the state of thetank and its associated pipework and valves.

[0007] If the test is being carried out in relatively steady atmosphericconditions, it is possible to derive some diagnostic information fromthe pressure within the tank alone. However, generally, it is preferablealso to monitor atmospheric conditions and to compensate the pressurewithin the tank to account for changes in atmospheric conditions.

[0008] The method can be used to determine the presence of a leak fromthe system as a whole. However, by isolating part of the tank and itsassociated pipework and valves and conducting a further test, moreinformation about the location of the leak can be obtained. The resultsof this test are preferably compared with the results of the test on theoverall system. Thus, if a valve is suspected of leaking, or if a leakis discovered in some other part of the system, this part of the systemcan be isolated and the test can be conducted on the remainder of thesystem to determine whether the remainder of the system now retains itspressure integrity.

[0009] A recent development is the introduction in certain fillingstations of vapour recovery pumps. These collect vapour from the vehiclefuel tank and pump it back into the underground storage tank. Such anarrangement distorts the readings from the above mentioned method. Underthese circumstances, preferably, the method further comprises disablingor blocking the vapour recovery pump prior to the monitoring step. Thus,the method is carried out without the vapour recovery pumps distortingthe readings.

[0010] A problem with the vapour recovery pumps is that the pressurereduction in the tank caused by the removal of the liquid hydrocarbonand the pressure increase within the tank caused by the vapour recoveryare never in perfect balance. If the depletion effect of the hydrocarbonremoval is dominant, the pressure in the tank will reduce until thelower limit of the P/V valve is reached. Each further sale will thenresult in air being drawn into the tank through the P/V valve. If, onthe other hand, the increase in pressure caused by the vapour recoverypumps is dominant, pressure within the tank will increase until theupper limit of the P/V valve is reached. Each further sale under thesecircumstances will cause enriched vapour to be released into theatmosphere. Therefore, it is desirable for the vapour recovery pumps tobe set to deliver less positive pressure into the tank than the negativeeffect caused by the removal of the liquid hydrogen. Thus, the methodpreferably comprises disabling or blocking all but one of the pumps inturn during the monitoring step. By doing this, it is possible toestablish for each hose a balance between the negative effect of theliquid hydrocarbon removal and the positive effect of the vapourrecovery. The vapour recovery pump will then be set accordingly.

[0011] A method in accordance with the present invention will now bedescribed with reference to the accompany drawings, in which:

[0012]FIG. 1 is a schematic representation of a system to which themethod is applicable;

[0013]FIG. 2 is a graph showing the pressure monitored with the P/Vvalve uncovered; and

[0014]FIG. 3 is a graph similar to FIG. 2 but showing pressure changeswith the P/V valve covered.

[0015]FIG. 1 represents a typical filling station. Fuel of variousdifferent grades is stored in underground tanks 1 only one of which isshown in FIG. 1. Each tank has its own fuel supply line 2, vapourrecovery line 3 and fuel outlet line 4. Each vapour recovery line 3 isconnected to a common manifold 5, the pressure of which is controlled bya pressure/vacuum (P/V) valve 6. The manifold 5 has an outlet 7 which isnormally closed by valve 8. The fuel outlet lines 4 from the tanks 1 areconnected to dispensing pumps 9 allowing a motorist to obtain fuel fromseveral different tanks.

[0016] In order to fill a tank 1, a delivery tanker is connected to thefuel inlet line 2 and to the outlet 7 of the manifold 5, in the processopening the valve 8. Fuel is then pumped into the tank 1 raising theliquid level 10 with the tank and driving the vapour along the vapourrecovery line 3 into manifold 5 and back into a vapour space in thetanker.

[0017] Leakage may occur from the system either through the walls of thetank 1 or the associated pipework 3,5 or the valves 6,8.

[0018] The present invention monitors the pressure without a deliverytanker being present and with the system operating normally to deliverfuel to the pumps 9. Under these circumstances, the fuel delivery line 2is closed and a pressure gauge is applied to the outlet 7 of themanifold 5. The pressure gauge will open the valve 8, but will itselfseal the outlet 7 of the manifold 5 so that there is no flow of vapourout of the manifold. Typical readings from the pressure gauge are shownin FIGS. 2 and 3. In these graphs, the following labelling is used:

[0019] A. Vehicle starts to draw fuel

[0020] B. Vehicle stops during fuel

[0021] X. P/V valve opens near design pressure

[0022] C. P/V valve closes near design pressure

[0023] Referring first to FIG. 2 as a motorist draws fuel through a pump9, the pressure in the system drops, as demonstrated by the downwardslope from A to B in FIG. 2. If the pressure drops below a certainlevel, the P/V valve opens as indicated at X in FIG. 2 to allow air intothe manifold 5. Once the motorist stops drawing fuel, the pressure risesdue to the flow of air through the P/V valve 6 as indicated by theupward slope from B to C. Once the pressure has reached a certain levelat C the P/V valve closes. From this point on, the pressure shouldremain constant until fuel is again drawn from pump 9. However, as canbe seen in FIG. 2, the pressure rises gradually prior to the next amountof fuel being drawn. This indicates the presence of a leak in thesystem. However, it is not apparent from FIG. 2 alone what the source ofthis leak is.

[0024] In order to obtain further information about this, the test isthen repeated with the P/V valve covered to produce the results shown inFIG. 3. Again, the pressure drops from A to B as fuel is drawn from thetank. However, under these circumstances, there is only a very gradualrise in the pressure from B to A while fuel is not being drawn. Thisindicates that the majority of the leakage was occurring through thevalve. However, the gradual increase indicates that there is still someleakage through the system. In the FIG. 3 as the P/V valve is covered,the pressure continues to drop as further fuel is drawn from the tank aspressure relief through the P/V valve is not available.

1. A method of testing the pressure integrity of a tank containinghydrocarbon in liquid and gaseous phase and its associated pipework andvalves, the tank having an inlet for the supply of liquid hydrocarbon tothe tank, a first outlet for the removal of gaseous hydrocarbon from thetank, and a second outlet for the removal of liquid hydrocarbon from thetank; the method comprising the steps of closing the inlet and firstoutlet and monitoring the pressure within the tank during normaloperation of the tank as liquid hydrocarbon is removed from the secondoutlet.
 2. A method according to claim 1, further comprising the step ofmonitoring atmospheric conditions and compensating the pressure withinthe tank to allow for changes and atmospheric conditions.
 3. A methodaccording to claim 1 or claim 2, further comprising the step ofisolating part of the tank and its associated pipework and valves andconducting a further test.
 4. A method according to any one of thepreceding claims, wherein a vapour recovery pump is provided to recovervapour from vehicle tanks which are being filled with liquid hydrocarbonby the second outlet, the method comprising disabling or blocking thevapour recovery pump prior to the monitoring step.
 5. A method accordingto claim 4, wherein a plurality of second outlets are provided and avapour recovery pump is associated with each outlet, the methodcomprising disabling or blocking all but one of the pumps in turn duringthe monitoring step.